﻿/* Copyright 2008 dnAnalytics Project.
 *
 * Contributors to this file:
 * Marcus Cuda - http://marcus.cuda.net
 *
 * This file is part of dnAnalytics.  dnAnalytics is licensed under the 
 * Microsoft Public License. See License.txt for a complete copy of the
 * license.
 */

using System;

namespace dnAnalytics.LinearAlgebra
{
    public partial class HouseholderTransformation
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="HouseholderTransformation"/> class.
        /// </summary>
        /// <param name="vector">The vector to transform.</param>
        public HouseholderTransformation(Vector vector)
        {
            if (vector == null)
            {
                throw new ArgumentNullException("vector");
            }
            mVector = vector.Clone();
            Compute();
        }

        //based on Golub Matrix Computations 3rd edition sections 5.1.3-5.1.4
        private void Compute()
        {
            Vector v = mVector.Clone();
            double sigma = 0;
            for (int i = 1; i < mVector.Count; i++)
            {
                sigma += mVector[i]*mVector[i];
            }

            if (sigma != 0)
            {
                double mu = System.Math.Sqrt(mVector[0]*mVector[0] + sigma);
                if (mVector[0] <= 0)
                {
                    v[0] = mVector[0] - mu;
                }
                else
                {
                    v[0] = -sigma/(mVector[0] + mu);
                }
                double s = (2.0*v[0]*v[0])/(sigma + (v[0]*v[0]));
                for (int i = v.Count - 1; i >= 0; i--)
                {
                    v[i] = v[i]/v[0];
                }
                double w = s*mVector.DotProduct(v);
                mBeta = mVector[0] - v[0]*w;
                mVector = v;
            }
        }
    }
}